Too Much Negative Pressure in Grow Tent [4 Ways to Fix]


While growing in a sealed grow regime, we have to mimic the mother nature for the plants. And one of the vital parts of the role is, to ensure a slight negative pressure across the garden. 

But what is there is too much negative pressure in a grow tent? Is it bad enough to destroy the dream of an optimal yield? Or you should just let it go anyway?

We have taken over to break this important issue in bite-size chunks. Throughout this article, we’ve noted why does a negative pressure takes place in a grow tent, how much of it is good enough, and how to overcome the excess negative pressure. The concern of too much negative pressure in grow room will also be under our radar. 

Got a minute to check out? Here you go-  

Why Negative Pressure Take Place in A Grow Tent?

Negative pressure indicates to have less air pressure inside an enclosed area comparing to the outside. A physics 101 is, when the air density inside an area is less than it’s surroundings, a negative air pressure takes place. As a result, the boundary of the enclosed area tends to pull inwards(concave).

So how does this apply in the growing regime? Well, when the rate of air leaving the grow tent(through exhaust fans) is more than the rate at which air enters(through intake fans) is higher, then you can say a negative pressure taking place in the grow tent. 

And these are the two situations where a negative pressure can take place- 

  • When the outtake fan is too much efficient than your intake fans. 
  • When the intake fan is drawing way less air in than what it’s supposed to do theoretically. This might take place when there are some large kinks(compression) in the intake ducting. 

If you want to learn how to get negative pressure in grow tent, playing with these two variables will get you the desired result. 

How Does Grow Tent Negative Pressure Affect on The Yield?

Slight negative pressure is good for maximizing the yield of a grow regime. It makes it easier to control the temperature, humidity, CO2 levels and other contaminants of the tent. That’s because when the hot and humid air exits the tent quickly, these consequences follow to drop down. 

Apart from these environmental benefits, there are some other benefits of slightly negative pressure in the grow tent- 

  1. Avoids the risk of mold or mildew formation on plants. 
  2. Lets you do only a few small atmospheric adjustments for different growth stages of the plants. 
  3. Keeps the unwanted odor under control and keeps from lingering in the tent. 

Having said that, is negative pressure good in a grow room? 

Well, too much of everything is always bad. And the same does for negative pressure as well. So, how would you understand if the negative pressure had exceeded the limit or not? 

The simple trick is- if the tent itself seems to pull itself inwards, the negative pressure is still under the tolerable limit. If the pressure gets as high as it bends the poles inwards, that’s where the danger limit starts. 

So, if you see the poles to bend inwards, the negative pressure is something to worry about. Otherwise, if it’s the tent itself if pulled inwards slightly, you don’t have to worry about it. 

4 Ways on How to Fix Negative Pressure in Grow Tent

1 of 4: Go for Active Intake Instead of Passive

In case you are drawing air in just through vent holes or flaps, that might not be sufficient enough to cope up with the high exhaust rate. Result? An increasing negative pressure.

Usually, tents of up to 4×4 size don’t really require an active intake fan. But for higher tent sizes, a passive intake grow tent would require. Now, the question is- what size intake fan should you go with? Here is a quick list(generalized)- 

For more on how we calculated the required fan size, read our fan size calculation guide

2 of 4: Get A Fan Speed Controller

When it comes to the fan size selection for a grow tent, there are some exact calculations. In case you have an exhaust fan with more power than what’s required, it will extract more air out than what is required. Result? A huge mismatch between the intake and exhaust fan size, and obviously, a negative pressure. 

So, getting a fan controller to control the CFM of the exhaust fan is a  way to fix negative pressure for grow tent. With the controller equipped with your exhaust fan, do some experiments on how much CFM is not creating too much negative pressure. Once found, set it at that level. Also, maintain a proper on-off timeline for the fans as well.

On an additional note, this tactic might work on too much negative pressure grow room as well. 

Our recommended fan speed controller-

  1. iPower GLFANXCONTROL (Best rated)
  2. Gotham Hydroponics Variable Fan Speed Controller (High-end)

3 of 4: Add An Extra Ducting Intake At the Bottom

Many growers deal with the negative air pressure issue just by adding one or more extra ducting intake at the bottom of the tent. In this way, you have to run a duct from the lower opening of the tent, and attach a cardboard box with an opening on 3 of its side. 

The purpose of such an arrangement is to make the ducting light-proof. The bigger the passive duct is(in diameter), the more air it can draw to overcome the negative pressure. Usually, we recommend going with 6” ducting intakes for tents with a ground area of 50 square feet or more. 

However, the length of the ducting should be that much with which you can create an ‘S’ shape. Such shape is scientifically effective to prevent any light entrance through it. Another idea to make it light-proof is, to bend it at   angle of 90 degrees right after the S-shape is created. 

To make the system more lightproof, here are some techniques to follow- 

  1. Using canvas layers(a few yards). 
  2. Using duct cloth. 
  3. Using gaffers tape. 
  4. By putting the duct openning in a dark(as dark as possible) side. 

4 of 4: Use A Light-proof Vent Flap

There are some tents which come with built-in ventilation flaps at the bottom of them. Opening up one of them can assist your intake fan to draw more fresh air and overcome the excess negative air pressure grow tent can’t tolerate. 

An important thing to check about the vent flaps is, it has to be lightproof anyway. Usually, some tents come with vent flaps that are made of light baffling materials. So the light entrance is minimal. 

But some other brands offer ‘just’ the vent flap without any light prevention. In that case, you can go with the idea pitched by Reddit user nazi-mod-mode’s comment

Keep a water bottle in front of the flap and allow the flap to close over the bottle. In case your tent is not located in a bright area, the bottle will be able to keep the lights out. 

However, if your tent is in a dark cycle during the day time(12 pm to 6 pm), this might not be a good idea as light rays can make their way into the tent. Otherwise, it’s okay to go with it if the tent is in a dark room. 

Bottom Line

There is now a particular meter or scale to measure how much negative pressure in grow tent is good. But if you check the poles out, you might get an indication of high negative pressure and take action. We have provided 4 of the actionable fixes, and you are supposed to pick up the right one that works for you. Good luck! 

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